We have identified a number of genes via genetic analysis and RNA interference gene inactivations that act as protein coding cofactors for the function of miRNAs and siRNAs in C. elegans. Some of these proteins were identified in genetic screens for decrease in miRNA function, some in genetic screens for decrease in siRNA function, and some in genetic screens for increase in siRNA function. We have also identified the target small RNAs that mediate these functions by deep RNA sequencing of selected mutant strains. We propose to dissect in detail how these proteins orchestrate the production, trafficking, and function of small RNAs in both mRNA degradation, mRNA translational control, and control of gene expression. We also propose to discern how the miRNA and siRNA and other small RNA pathways may compete with each other for common cofactors, thus leading to an increase in function in one pathway, when the other pathway is debilitated. The genes identified in this study are likely to be key factors in the function of small RNAs in biology and thus their identification may enable more potent RNAi based drug development. PUBLIC HEALTH RELEVANCE: The Ruvkun lab has identified a large collection of genes that act as cofactors for the function of small RNA genes in C. elegans. We propose to dissect in detail how these proteins orchestrate the production, trafficking, and function of small RNAs in control of gene expression. The genes identified in this study are likely to be key factors in the function of small RNAs in biology and thus their identification may enable more potent RNA interference-based drug development.